test(ci): harden two flaky tests against CI noise (#33675)

Two unrelated transient failures on PR #33661's initial CI run, both
pre-existing on main and recovered on rerun. Hardening:

1. tests/cron/test_scheduler.py::TestRunJobConfigLogging — added mocks for
   resolve_runtime_provider() and discover_mcp_tools(). The yaml-warning
   tests intend to exercise only the warning-log path, but
   _run_job_impl continues into provider resolution and MCP discovery
   after the warning. Both can spawn subprocesses / hit the network and
   pushed the test over its 30s budget under GHA load.

2. tests/tools/test_browser_supervisor.py — wrapped Chrome teardown
   against the stdlib subprocess._wait() race (bpo-38630). When SIGCHLD
   arrives during proc.wait(), _try_wait(WNOHANG) can return a foreign
   pid and the 'assert pid == self.pid or pid == 0' fires. Fixture now
   catches AssertionError/TimeoutExpired, force-kills, and always reaps
   so no zombie escapes. Same hardening applied to the early-skip branch.

tests/tools/test_browser_supervisor.py

@@ -89,18 +89,45 @@ def chrome_cdp(request):
         except Exception:
             time.sleep(0.25)
     if ws_url is None:
-        proc.terminate()
-        proc.wait(timeout=5)
+        try:
+            proc.terminate()
+            proc.wait(timeout=5)
+        except (subprocess.TimeoutExpired, AssertionError, Exception):
+            try:
+                proc.kill()
+            except Exception:
+                pass
+            try:
+                proc.wait(timeout=2)
+            except (AssertionError, Exception):
+                pass
         shutil.rmtree(profile, ignore_errors=True)
         pytest.skip("Chrome didn't expose CDP in time")
 
     yield ws_url, port
 
-    proc.terminate()
+    # Tear down Chrome. The stdlib `subprocess._wait()` POSIX implementation
+    # has a known race (https://bugs.python.org/issue38630): when SIGCHLD
+    # arrives concurrently with `proc.wait()`, `_try_wait(WNOHANG)` can
+    # return a foreign pid and the `assert pid == self.pid or pid == 0`
+    # fires. We saw this in CI on slice 1 after this fixture's teardown
+    # (PR #33661 follow-up). Swallow the stdlib race + force-kill if wait
+    # hangs, then always reap so we don't leak a zombie.
+    try:
+        proc.terminate()
+    except Exception:
+        pass
     try:
         proc.wait(timeout=3)
-    except Exception:
-        proc.kill()
+    except (subprocess.TimeoutExpired, AssertionError, Exception):
+        try:
+            proc.kill()
+        except Exception:
+            pass
+        try:
+            proc.wait(timeout=2)
+        except (AssertionError, Exception):
+            pass
     shutil.rmtree(profile, ignore_errors=True)